1. Field of the Invention
The present invention relates to a gas sensor including a hydrogen sensor for measuring or detecting a hydrogen component in a gas of interest, in particular, to a hydrogen sensor for measuring concentration of hydrogen contained in a fuel gas for use in a fuel cell.
2. Description of the Related Art
In response to concerns about global environmental pollution, in recent years intensive studies have been conducted on fuel cells for use as high-efficiency, clean power sources. Among such fuel cells, a polymer electrolyte fuel cell (PEFC) shows promise for automobile use and household use, by virtue of its inherent advantages, such as operation at low temperature and high output density.
A promising fuel gas for use in PEFC is a reformed gas. In this connection, in order to enhance efficiency and the like factor, a sensor capable of directly detecting hydrogen in a reformed gas must be provided. Since this sensor is used in a hydrogen rich atmosphere, an operating temperature thereof must be low (about 100xc2x0 C. or lower).
Such low-temperature operation type sensor is proposed in, for example, European Patent No. 1103807A2. As shown in FIG. 5, the proposed sensor employs a proton conductive layer 101 formed from a polymer electrolyte and is configured such that a first electrode 102 and a second electrode 103 are disposed on the corresponding surfaces of the proton conductive layer 101, and the resultant assembly is held between a pair of support elements 104 and 105.
In the above-mentioned sensor, an upper support 104 includes a diffusion limiting portion 106 for diffusion of gas while establishing communication between the first electrode 102 and an outside atmosphere containing a gas to be measured, and the other support 105 includes a gas outlet portion 107 for releasing hydrogen from the sensor while establishing communication between the atmosphere and the second electrode 103. The hydrogen concentration can be measured on the basis of the limiting current flowing between the first and second electrodes 102,103.
3. Problems Solved by the Invention
However, the inventors have found a drawback that under certain conditions, the above mentioned conventional sensor has generated an undesirable output. Specifically, an abruptly varying concentration of hydrogen gas has caused transient generation of unusual electromotive force (or rather, undesired undershooting current) between the first electrode 102 and the second electrode 103, as shown in FIG. 2. As a result, during the transition, it is difficult or rather impossible to use the sensor for measuring hydrogen gas concentration.
The present invention has been achieved in order to solve the above-mentioned problem of the prior art, and an object of the present invention is to provide an improved hydrogen sensor capable of accurately measuring gas concentration even during transition.
A hydrogen sensor according to the present invention has the following major feature. That is, a gas diffusion-limiting outlet (21) provided between a second electrode (5) and a measurement gas (6) has a predetermined gas diffusion resistance which limits diffusion of hydrogen molecules entering or draining out through the outlet, when a voltage is applied across the first electrode (3) and the second electrode (5) as shown in FIG. 1. The gas diffusion-limiting outlet (21) may be formed in a support element (9) that supports a proton conductive layer (1) and encapsulates the second electrode formed on the proton conduction layer (1).
Specifically, the diffusion resistance (b) of the gas diffusion outlet (21) is so predetermined that a ratio (a/b) of the gas diffusion resistance (a) of the gas diffusion-limiting inlet (19) to the gas diffusion resistance (b) of the gas diffusion-limiting outlet (21) is not greater than 2. In other words, the gas diffusion-limiting inlet (19) is designed to have a diffusion resistance (a) of not more than two times the diffusion resistance (b) of the gas diffusion-limiting outlet (21).
More specifically, the ratio (a/b) that is the gas diffusion resistance (a) of the gas diffusion-limiting inlet (19) to the gas diffusion resistance (b) of the gas diffusion-limiting outlet (5) is from 1 to 2.
With the above feature incorporated into the hydrogen sensor, the problem of undershooting current is effectively or completely suppressed. When the diffusion resistance ratio (a/b) falls within the above range, even upon abrupt increase in the hydrogen concentration of the measurement gas, the amount of the hydrogen gas which reaches the second electrode (5) through the gas diffusion limiting outlet (21) can be rendered closer to that of the hydrogen gas which reaches the first electrode (3) through the diffusion limiting inlet (19), whereby generation of unusual electromotive force or undershooting current can be effectively suppressed. The unusual electromotive force or undershooting current herein means more than 10% of the value thereof produced from the value that correctly represents the concentration, resulting in a measurement error.
In short, the hydrogen gas entering and/or draining out of the sensor according to the present invention is effectively and stably controlled by the two diffusion resistances having close values formed respectively in the first and second support elements (3,5), one formed at the diffusion limiting inlet (19) and the other at the diffusion limiting outlet (21), whereby accurate concentration measurement of hydrogen gas contained in a hydrogen-component varying atmosphere is attained.
The present invention is applicable to both a hydrogen sensor having no reference electrode as shown in FIG. 1 and a hydrogen sensor having a reference electrode (37) as shown in FIG. 4. In the latter sensor, the voltage applied across the first and second electrodes (33,35) for measuring the limiting current that flows between the first and second electrodes (33,35) is varied while the voltage applied across the first electrode (33) and the reference electrode (37) is maintained constant, so that a wider range of the limiting current (resulting in a wider range of the hydrogen concentration) is attained, compared to the hydrogen sensor without a reference electrode.
In designing the diffusion resistance ratio (a/b) to fall in the range of 1-2 by forming through-holes in the support elements, a dimensional ratio (de/cf) is set to the range of 1-2, where (c) is a cross sectional area of a first through-hole formed as a diffusion limiting inlet (19), (d) is a length of the first through-hole, (e) is a cross sectional area of a second through-hole formed as a diffusion limiting outlet (21) and (f) is a length of the second through-hole.
The diffusion resistance ratio (a/b) between the diffusion resistance of the diffusion-limiting inlet (19) and the diffusion resistance of the diffusion-limiting outlet (21) can be confirmed by measuring two limiting currents of the hydrogen sensor placed in a gas having a predetermined hydrogen concentration, since the limiting current is proportionally dependent on the reciprocal of the diffusion resistance. A first limiting current limited by the diffusion resistance of the diffusion limiting inlet (19) is determined by applying a predeternmined dc voltage across the first and second electrodes (3, 5) as shown in FIG.1, and the second limiting current limited by the diffusion resistance of the diffusion limiting outlet (21) is determined by reversing the polarity of the dc voltage. The diffusion resistance ratio (a/b) is, therefore, identified by the second limiting current divided by the first limiting current.